If a plurality of DC/AC converters are connected on their input side via a respective disconnector to common bus lines, then in the case of a serious fault in one of the DC/AC converters that results in a short circuit between its connection lines, the short-circuit current generated by all photovoltaic generators connected to the bus lines flows via the disconnector provided between the bus conductor and the faulty DC/AC converter. In order to be able to safely interrupt the short-circuit current with the disconnector, the limiting turn-off capacity of the disconnector has to be designed for the short-circuit current, even though the latter is a multiple of the DC current that flows via the disconnector in undisturbed operation and has to be turned off by means of the disconnector.
Fuses, via which the photovoltaic generators might be connected to the bus lines, usually do not respond to the short-circuit current flowing in the described case because the short-circuit current is only slightly greater than the nominal current of the respective photovoltaic generator.
DE 10 2010 017 746 A1 discloses a photovoltaic installation comprising a photovoltaic generator connected to a photovoltaic inverter. The photovoltaic inverter comprises a DC disconnecting device that is controllable by means of a control device. Furthermore, the photovoltaic inverter comprises a short-circuiting device, by means of which the photovoltaic generator can be short-circuited upstream of the DC disconnecting device and which is likewise controllable by means of the control device. The DC disconnecting device can comprise a relay that causes all-pole disconnection of the photovoltaic generator from a DC/AC converter of the photovoltaic inverter. The short-circuiting device can comprise a semiconductor switch.
DE 10 2009 019 831 A1 discloses a circuit arrangement for electrically coupling a DC source, e.g. a photovoltaic generator, to a DC load, e.g. an inverter. The circuit arrangement comprises a first and second current input node for receiving a DC current generated by the DC source, and a first and second current output node for outputting the generated DC current to the DC load. The circuit arrangement also includes a disconnecting means connected between the first current input node and the first current output node and/or the second current input node and the second current output node and serving for interrupting the electrical connection between the respective nodes, and an auxiliary switching means connected between the first current input node and the second current input node and serving for short-circuiting the first and second current input nodes. If the auxiliary switching means for short-circuiting is closed, the DC source is thus short-circuited. What can be achieved thereby is that the current to the load is reduced to zero. Although the DC source continues to generate DC current, the latter no longer flows, or flows at most in very small amounts, to the load due to the short circuit. The disconnecting means can then open in order to interrupt the electrical connection between the respective nodes. Consequently, the known problem of interrupting even a particularly large DC current does not occur for the disconnecting means. Rather, it can switch in the currentless state. Accordingly, it is possible to use a cheap disconnecting means, which does not need to be suitable for switching under load. DE 10 2009 019 831 A1 is not concerned with the question of safely turning off the short-circuit current by the auxiliary switching means.
WO 2011/023732 A2 discloses a bypass and protection circuit for a solar module and a method for controlling the solar module. The bypass and protection circuit comprises an input for connecting the solar module, an output, a bypass element connected in parallel with the output, a disconnecting element connected between the input and the output, and a protection element in the form of a diode connected in parallel with the input. In this case, the disconnecting element is configured to control a connection between the input and the output depending on whether the solar module assigned to the bypass and protection circuit is wholly or partly shaded or is intended to be switched on or off. The disconnecting element comprises a switch, and the bypass element comprises a diode with a switch connected in parallel. The bypass element is arranged downstream of the disconnecting element, as viewed from the solar module. Relays can be used as the switch. Semiconductor components are preferred. In the case of the connected solar module being shaded, the switch of the disconnecting element is opened and the switch of the bypass element is closed.
WO 2004/082091 A1 discloses an electronic disconnector for connecting a power supply network to an output for connecting a load. Between the input and the output, the disconnector comprises a series connection of a fuse, a semiconductor switch and a relay. The controller controls the semiconductor switch in such a way that substantially voltageless switching of the relay occurs.
DE 10 2010 007 452 A1 discloses a load relief for a disconnector for use in the field of electric vehicles, wherein the disconnector has to perform a galvanic isolation between a battery and an intermediate circuit. The load relief comprises at least one semiconductor switch. For disconnecting the electrical connection, the current to be turned off is conducted via the semiconductor switch. The disconnector is then turned off under reduced voltage increase.
EP 2 148 417 A1 discloses an inverter circuit arrangement for a photovoltaic generator comprising a plurality of DC/AC converters connected in series at the input. A photovoltaic generator is connected to this input-side series connection of the DC/AC converters. A bridging switch is provided for each of the DC/AC converters. The bridging switch is located in a DC voltage intermediate circuit and bridges the respective DC/AC converter in the closed state. A resistance chopper is connected in parallel with each bridging switch. Each resistance chopper consists of a controllable semiconductor switch, a resistor connected in series therewith, and a diode connected in parallel with the resistor, wherein the forward direction of the diode is in the opposite direction to that semiconductor switch, such that it fulfills the function of a freewheeling diode. With bridging switches open, the resistance choppers are clocked in order to discharge capacitors of input-side DC voltage intermediate circuits of the DC/AC converters.